feat(altair): implement scatter-marginal (#6128)

## Implementation: `scatter-marginal` - python/altair

Implements the **python/altair** version of `scatter-marginal`.

**File:** `plots/scatter-marginal/implementations/python/altair.py`

**Parent Issue:** #2005

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25592784974)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/scatter-marginal/implementations/python/altair.py

@@ -1,14 +1,32 @@
-""" pyplots.ai
+""" anyplot.ai
 scatter-marginal: Scatter Plot with Marginal Distributions
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 91/100 | Created: 2025-12-26
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 94/100 | Updated: 2026-05-09
 """
 
-import altair as alt
-import numpy as np
-import pandas as pd
+import os
+import sys
+from pathlib import Path
 
 
+current_dir = Path(__file__).parent
+sys.path = [p for p in sys.path if p != str(current_dir)]
+
+import altair as alt  # noqa: E402
+import numpy as np  # noqa: E402
+import pandas as pd  # noqa: E402
+
+
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+ELEVATED_BG = "#FFFDF6" if THEME == "light" else "#242420"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+
+# Okabe-Ito palette
+BRAND = "#009E73"
+
 # Data - bivariate distribution with correlation
 np.random.seed(42)
 n = 150
@@ -21,40 +39,47 @@
 x_domain = [df["X Value"].min() - 2, df["X Value"].max() + 2]
 y_domain = [df["Y Value"].min() - 2, df["Y Value"].max() + 2]
 
-# Interactive brush selection
-brush = alt.param(name="brush", select="interval")
-
 # Base chart
 base = alt.Chart(df)
 
-# Main scatter plot with selection
+# Main scatter plot with grid
 scatter = (
-    base.mark_circle(size=120, opacity=0.65)
+    base.mark_circle(size=120, opacity=0.65, color=BRAND)
     .encode(
         x=alt.X("X Value:Q", title="X Value (units)", scale=alt.Scale(domain=x_domain)),
         y=alt.Y("Y Value:Q", title="Y Value (units)", scale=alt.Scale(domain=y_domain)),
-        color=alt.condition(brush, alt.value("#306998"), alt.value("lightgray")),
         tooltip=["X Value:Q", "Y Value:Q"],
     )
     .properties(width=1000, height=600)
-    .add_params(brush)
 )
 
 # Top marginal histogram with matching X scale
 top_hist = (
-    base.mark_bar(color="#306998", opacity=0.5)
+    base.mark_bar(color=BRAND, opacity=0.5)
     .encode(
-        x=alt.X("X Value:Q", bin=alt.Bin(maxbins=25), title=None, scale=alt.Scale(domain=x_domain), axis=None),
+        x=alt.X(
+            "X Value:Q",
+            bin=alt.Bin(maxbins=25),
+            title=None,
+            scale=alt.Scale(domain=x_domain),
+            axis=alt.Axis(labels=False, ticks=False),
+        ),
         y=alt.Y("count()", title=None, axis=alt.Axis(labels=False, ticks=False)),
     )
     .properties(width=1000, height=120)
 )
 
 # Right marginal histogram with matching Y scale
 right_hist = (
-    base.mark_bar(color="#306998", opacity=0.5)
+    base.mark_bar(color=BRAND, opacity=0.5)
     .encode(
-        y=alt.Y("Y Value:Q", bin=alt.Bin(maxbins=25), title=None, scale=alt.Scale(domain=y_domain), axis=None),
+        y=alt.Y(
+            "Y Value:Q",
+            bin=alt.Bin(maxbins=25),
+            title=None,
+            scale=alt.Scale(domain=y_domain),
+            axis=alt.Axis(labels=False, ticks=False),
+        ),
         x=alt.X("count()", title=None, axis=alt.Axis(labels=False, ticks=False)),
     )
     .properties(width=120, height=600)
@@ -63,13 +88,24 @@
 # Combine: top histogram above, scatter with right histogram below
 combined = (
     alt.vconcat(top_hist, alt.hconcat(scatter, right_hist, spacing=5), spacing=5)
-    .properties(title=alt.Title(text="scatter-marginal · altair · pyplots.ai", fontSize=28, anchor="middle"))
-    .configure_axis(labelFontSize=16, titleFontSize=20)
-    .configure_title(fontSize=28)
-    .configure_view(strokeWidth=0)
+    .properties(
+        background=PAGE_BG,
+        title=alt.Title(text="scatter-marginal · altair · anyplot.ai", fontSize=28, anchor="middle", color=INK),
+    )
+    .configure_axis(
+        domainColor=INK_SOFT,
+        tickColor=INK_SOFT,
+        gridColor=INK,
+        gridOpacity=0.10,
+        labelColor=INK_SOFT,
+        labelFontSize=16,
+        titleColor=INK,
+        titleFontSize=20,
+    )
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT, strokeWidth=1)
     .configure_concat(spacing=5)
 )
 
 # Save outputs
-combined.save("plot.png", scale_factor=3.0)
-combined.save("plot.html")
+combined.save(f"plot-{THEME}.png", scale_factor=3.0)
+combined.save(f"plot-{THEME}.html")